1. Field of the Invention
This invention relates to solid state devices and more particularly to solid state devices for converting energy between electromagnetic and electrical energy such as light emitting device and photovoltaic device.
2. Background of the Invention
A variety of semiconductors including GaN (Gallium Nitride), GaP (Gallium Phosphide), AlGaAs (Aluminum Gallium Arsenide), InP (Indium Phosphide), Si (Silicon) and SiC are currently being studied for light emitting diode (LED) applications. Recently, efficient luminescence has been discovered in silicon, which is an indirect bandgap semiconductor, due to alteration of its defect states [Wai Lek Ng, et al, Nature, Vol. 410, Issue 8 (2001)]. However in SiC, an indirect bandgap wide-bandgap semiconductor (WBGS) generally emitting in the blue spectra, no efficient radiative isoelectronic trap has been discovered; consequently development of this LED material has languished. Other recent papers show that SiC LEDs have potential for high quantum efficiency [a) S. I. Vlaskina, “Silicon Carbide LED”, Semiconductor Physics, Quantum Electronics & Optoelectronics, 2002, Vol. 5, No. 1, pp 71-75, b) S. Kamiyama et al, “Extremely high quantum efficiency of donor-acceptor-pair emission in N- and B-doped 6H-SiC, JAP, 99, 093108 (2006).].
P-type doping, particularly exhibiting low resistivity, has been a drawback for wide-bandgap semiconductor LED material candidates. Ion implantation and thermal diffusion are currently the most widely used techniques for doping semiconductors. The energy scattered by the decelerating ions leads to crystal damage by introducing point defects and the formation of amorphous material. Post ion implantation high temperature annealing isothermally heats the wafer inducing undesirable dopant diffusion in previously prepared under layers.
Doping is one of the challenges for wide bandgap semiconductors, particularly SiC, device fabrication due to its hardness, chemical inertness and the low diffusion coefficient of most impurities I. A. Salama, Ph.D. Dissertation, University of Central Florida, Spring (2003). Current doping methods used for SiC device fabrication include epilayer doping and ion implantation. Maximum doping concentration is limited by the solubility dopant depth is limited by the mass diffusion coefficient of the dopant in the wide bandgap semiconductor substrate. Epilayer doping is in situ doping during chemical vapor deposition epitaxial growth and is limited in SiC to nitrogen (N) or phosphorous (P) for n-type, aluminum (Al) or boron (B) for p-type and vanadium (V) for semi-insulating type. Ion implantation is the most common doping technique used used for wide bandgap semiconductor devices. This process generates implantation-induced defect centers including amorphitization in the wafer and requires high annealing temperatures to remove these defects and to electrically activate the dopants. In SiC some defects remain after annealing at temperatures up to 1700° C. [Z. Tian., N. R. Quick. and A. Kar, Acta Materialia, Vol. 53, (2005), pp. 2835-2844]. Annealing at these temperatures can cause severe surface damage due to silicon sublimation and redistribution [Z. Tian, N. R. Quick and A. Kar, Acta Materialia, 54, 4273, (2006)]. In summary conventional doping processes limit the dopant species, dopant concentrations and create defects.
Photolithographic patterning is necessary to define the areas across the sample to be selectively doped. This usually requires up to 10-15 individual processing steps. Damage assisted sublimation etching (DASE) of Si and dopant out-diffusion are common problems observed during ion implantation of SiC. Techniques allowing direct doping without the requirement for prepatterning can become economically viable [a) S. J. Pearton, Processing of Wide Bandgap Semiconductors, 1st Edition, William Andrew Publishing, 2000; b) Z. C. Feng, J. H. Zhao, “Silicon Carbide: Materials, Processing, Devices”, Optoelectronic Properties of Semiconductors and Superlattices, vol. 20, Taylor and Francis Books, Inc., 2004; c) M. E. Levinshtein et al, “Properties of Advanced Semiconductor Materials”, Wiely-Interscience Publications, 2001].
U.S. Pat. No. 5,063,421 to Suzuki et al. discloses a silicon carbide light emitting diode having a p-n junction is disclosed which comprises a semiconductor substrate, a first silicon carbide single-crystal layer of one conductivity formed on the substrate, and a second silicon carbide single-crystal layer of the opposite conductivity formed on the first silicon carbide layer, the first and second silicon carbide layers constituting the p-n junction, wherein at least one of the first and second silicon carbide layers contains a tetravalent transition element as a luminescent center.
U.S. Pat. No. 5,243,204 to Suzuki et al. discloses a silicon carbide light emitting diodes having a p-n junction which is constituted by a p-type silicon carbide single-crystal layer and an n-type silicon carbide single-crystal layer formed thereon. In cases where light emission caused by recombination of free excitons is substantially utilized, at least a part of the n-type silicon carbide layer adjacent to the interface of the p-n junction is doped with a donor impurity at a concentration of 5×1016 cm-3 or lower. In cases where light emission caused by acceptor-associated recombination is substantially utilized, the p-type silicon carbide layer is doped with an acceptor impurity and at least a part of the n-type silicon carbide layer adjacent to the interface of the p-n junction is doped with a donor impurity at a concentration of 1×1018 cm-3 or higher. Also provided are a method for producing such silicon carbide light emitting diodes and a method for producing another silicon carbide light emitting diode.
U.S. Pat. No. 5,416,342 to Edmond et al. discloses a light emitting diode is disclosed that emits light in the blue portion of the visible spectrum with high external quantum efficiency. The diode comprises a single crystal silicon carbide substrate having a first conductivity type, a first epitaxial layer of silicon carbide on the substrate and having the same conductivity type as the substrate, and a second epitaxial layer of silicon carbide on the first epitaxial layer and having the opposite conductivity type from the first layer. The first and second epitaxial layers forming a p-n junction, and the diode includes Ohmic contacts for applying a potential difference across the p-n junction. The second epitaxial layer has side walls and a top surface that forms the top surface of the diode, and the second epitaxial layer has a thickness sufficient to increase the solid angle at which light emitted by the junction will radiate externally from the side walls, but less than the thickness at which internal absorption in said second layer would substantially reduce the light emitted from said top surface of the diode.
U.S. Pat. No. 6,900,465 to Nakamura et al. discloses a nitride semiconductor light-emitting device has an active layer of a single-quantum well structure or multi-quantum well made of a nitride semiconductor containing indium and gallium. A first p-type clad layer made of a p-type nitride semiconductor containing aluminum and gallium is provided in contact with one surface of the active layer. A second p-type clad layer made of a p-type nitride semiconductor containing aluminum and gallium is provided on the first p-type clad layer. The second p-type clad layer has a larger band gap than that of the first p-type clad layer. An n-type semiconductor layer is provided in contact with the other surface of the active layer.
U.S. Pat. No. 6,998,690 to Nakamura et al discloses a gallium nitride-based III-V Group compound semiconductor device has a gallium nitride-based III-V Group compound semiconductor layer provided over a substrate, and an Ohmic electrode provided in contact with the semiconductor layer. The Ohmic electrode is formed of a metallic material, and has been annealed.
U.S. Pat. No. 7,045,375 to Wu et al. discloses a white-light emitting device comprising a first PRS-LED and a second PRS-LED. The first PRS-LED has a primary light source to emit blue light and a secondary light source to emit red light responsive to the blue light; and the second PRS-LED has a primary light source to emit green light and a secondary light source for emitting red light responsive to the green light. Each of the primary light sources is made from an InGaN layer disposed between a p-type GaN layer and an n-type GaN layer. The secondary light sources are made from AlGaInP. The primary light source and the secondary light source can be disposed on opposite sides of a sapphire substrate. Alternatively, the second light source is disposed on the n-type GaN layer of the primary light source. The second light sources may comprise micro-rods of AlGaInP of same or different compositions.
Discussion of wide bandgap materials and the processing thereof are discussed in U.S. Pat. No. 5,145,741; U.S. Pat. No. 5,391,841; U.S. Pat. No. 5,793,042; U.S. Pat. No. 5,837,607; U.S. Pat. No. 6,025,609; U.S. Pat. No. 6,054,375; U.S. Pat. No. 6,271,576, U.S. Pat. No. 6,670,693, U.S. Pat. No. 6,930,009 and U.S. Pat. No. 6,939,748 are hereby incorporated by reference into the present application.
Therefore, it is an objective of this invention to provide a solid state energy conversion device and method of making a light emitting device to produce electromagnetic radiation upon the application of electrical power and/or a photovoltaic device to produce electrical power upon the application of electromagnetic radiation.
Another objective of this invention is to provide a solid state energy conversion device and method of making in a wide bandgap semiconductor for enabling operation of the solid state energy conversion device at elevated temperatures.
Another objective of this invention is to provide a solid state energy conversion device and method of making that reduces the materials and layers required to fabricate the solid state energy conversion device through the fabrication of a monolithic structure.
Another objective of this invention is to provide a solid state energy conversion device and method of making that incorporates a process for doping conventional and unconventional dopants in indirect wide bandgap semiconductors to create efficient radiative states to provide an efficient solid state energy conversion device.
Another objective of this invention is to provide a solid state energy conversion device and method of making that incorporates combinations of dopants which enables the tuning of solid state energy conversion device to a white light sensitivity of the solid state energy conversion device.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed as being merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by modifying the invention within the scope of the invention. Accordingly other objects in a full understanding of the invention may be had by referring to the summary of the invention, the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.